(1) Technical Field
This invention relates to kinetic bonding of metal tubes to a metal tubesheet.
(2) Description of the Prior Art
The bonding of metal tubes to a metal tubesheet is an essential procedure in fabricating many types of heat exchangers. In steam condensers of the surface type, which is the type of heat exchanger most often used in systems for generating electric power from fossil fuel, geothermal and nuclear energy sources, the quality of the bond between tubes and a tubesheet is especially important. A steam condenser of the surface type is a heat exchanger in which cooling water is circulated through tubes whose exterior surfaces are exposed to steam that is to be condensed.
Bond failure between tubes and a tubesheet in any type of heat exchanger, particularly in a steam condenser used in an electric power generating system, could result in costly damage to equipment and costly downtime. Regardless of the application or operating environment of the heat exchanger, high-quality bonding between the tubes and the tubesheet is of considerable importance in minimizing corrosion.
Conventional welding techniques for bonding a plurality of individual tubes to a tubesheet are quite time consuming. Furthermore, conventional welding techniques in general cannot be used for bonding tubes to tubesheets that are made of dissimilar metals. Consequently, techniques for explosively bonding tubes to tubesheets have been developed. Various techniques for explosively bonding individual metal tubes to a metal tubesheet have been described in the patent literature: e.g., U.S. Pat. Nos. 3,503,110; 3,698,067; 3,717,925; 3,774,291; 3,993,001; 4,003,513; 4,117,966; and 4,205,422. A technique was also described in U.S. Pat. No. 3,993,001 for bonding an array of tubes to a tubesheet in a single operation by initiating simultaneous detonations of explosive charges in all the tubes of the array.
With some kinetic bonding techniques in which explosive charges were detonated inside tubes positioned in bores through a tubesheet, crevices tended to be formed at the front face of the tubesheet between the exterior surfaces of the tubes and the surrounding surfaces of the bores. Such crevices were potential sites for corrosion, which could weaken and ultimately cause failure of the bonds. An expedient developed by Westinghouse Electric Corporation to minimize front face crevicing involved securing a disposable metal plate fixture over the front face of the tubesheet during detonation of the explosive charges. Such an expedient, however, introduced the problem of controlling tolerances in manufacturing the disposable metal fixturing.
A need has existed in the prior art for a highly reliable explosive bonding technique for metallurgically bonding an array of tubes to a tubesheet at a high rate of production appropriate for commercial manufacturing operations without causing appreciable crevicing at the front face of the tubesheet.